Last month's 3GSM World Congress in Barcelona was a watershed event for fixed/mobile telecom convergencewherein users with one phone device (and one number) can take advantage of inexpensive high-speed connectivity in the fixed-line home or office setting and fall back on the mobile network while out and about.

Chicago-based BridgePort Networks announced two separate demonstrations at the show, in which calls were handed off between GSM and IP-based voice over wireless LANone with smartphone vendor E28 Limited, one with communications software vendor PCTEL, using a handheld computer.

These three vendors are all members of MobileIGNITE, a trade association that originated as a BridgePort partner program but has since spun off as an independent organization whose members share a common architectural approach to achieving true F/MC.

We looked at the client side of this architecture last week. Now it's time to delve into BridgePort's contribution, the IMS (IP multimedia subsystem) Convergence Server (or ICS), and how the system functions. (IMS is a standards-based next-generation architecture that will let telecom operators offer both fixed and mobile services over circuit-switched and packet-based networks alike.)

Sanjay Jhawar, senior vice president of marketing and business development at BridgePort, initially pointed out to VoIPplanet.com that there are actually two versions of the ICS product, one for IMS-compliant networks, and one for current, pre-IMS networks. "IMS is a system that is in the early stages of deployment and will probably take the next three to four years to find its way into a majority of networks," he said. Nevertheless, it is definitely "a strategic direction that most of these service provider networks are going in," he continued, pointing out that it was strategic for BridgePort to be able to address both cases.

Unlike some other F/MC architectures, which are fundamentally mobile operator-only plays, BridgePort 's architecture will work with any kind of network transport. The ICS is a software-only network element, typically running in the service provider's network. "It can sit in a mobile operators network, a broaband operator's network, an ISP. Or it could sit somewhereon a hosted basisin between the two. And we have examples of all those models," Jhawar said.

Opting to describe the architecture of the 'current network' version of ICS (since that's what most customers will be using for the foreseeable future), Jhawar told VoIPplanet that, regardless of where it sits, the ICS looks to the host network like a Mobile Switching Center (MSC)specifically a Visiting Location Register MSC (a temporary database of subscribers who have roamed into the core network's area). When the SIP (voice over Wi-Fi) portion of the dual-mode phone is on (available to the ICP via the IP network), it registers to the ICS platform, which in turn registers it into the Home Location Register, essentially as a roaming visitor.

"Any call made to the mobile phone number in question goes to the mobile switch, and it will then get routed to our platform," Jhawar explained. At this point, the ICS initiates a SIP-over-broadband route to the phone, rings the phone, and controls the media path so that the media can get converted in the media gateway from circuit to packet, and reach the phone. "That's how you establish a basic call to a mobile phone number and terminate it on Wi-Fi," Jhawar concluded.

In making an outbound call from the SIP/IP side, the process is essentially reversed. "Assuming we're deployed on behalf of a mobile operator," Jhawar explained, "it will go as a SIP IP call to our box. At that point it is in the trusted mobile network, but it won't go into the cellular network; it will go straight to the core network. It won't go through any towers, it won't go through any circuit-switched backhaul, it won't go to any base station contollers."

Jhawar pointed out that, depending on how things were set up with the mobile operator, other possibilities exist, such as routing some calls directly to the PSTN, or to another SIP endpoint via ENUM. "But the key point is it bypasses the circuit switched backhaul, which is most of the cost of the mobile network."

It's a key point because a major part of the rationale for F/MCat least from the service provider's point of viewis to bring down costs, and pass at least some of those cost savings along to customers.

Jhawar reminded VoIPplanet that the BridgePort/MobileIGNITE architecture is not limited to mobile phone operators. BridgePort is working with cable MSOs, IP providers, and ISPs functioning as MVNOs, who may have other routing arrangements. "We'll run it wherever anybody wants," he said. "The technology supports whatever business model the provider would like to offer."

In fact, although BridgePort is in customer trials with a number of carriers all over the world, one of the few commercial deployments already up and running is with VeriSign. This is the hosted example, mentioned earlier. "VeriSign operates an inter-carrier SS7 network," Jhawar explained, "and it's starting to do VoIP peering. They're already connected up to most of the mobile carriers and many of the IP providers as well, so for a service provider that doesn't want to have to get into deploying their own infrastructure, they could buy it on a hosted basis from VeriSign," he said.

One BridgePort F/MC trial that's been announced is in conjunction with CDMA operator China Unicom in Guangdong province. There, the roaming will not be to Wi-Fi on a dual-mode phone, but rather to softphones on broadband PC. As to impending F/MC deployments, Jhawar said, "we may see two to three commercial launches this year; well see five or six more next year, based on our platform."

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